I received a phone call the other day that caught me a bit off guard. A customer was asking what tolerance they need to have on their motor shaft for it to fit with one of our incremental encoders. I reflexively told the customer that we machine our shaft I.D. to a –0.0000” +.0005” tolerance. He told me he already knew that, but what did we recommend for his shaft, the motor shaft that the incremental encoder was to be mounted on?

Since I am an Electrical Engineer, I wanted to make sure I had all of my ducks in a row before I took my second shot at answering a mechanical related question. I let him know that I would look up the information he needed and call him back. After I got off the phone I immediately knew what I should have told him. This answer may surprise you, but we don’t specify tolerances for the fit of our customer’s motor shafts. – not in the way one would expect. Instead of a fit tolerance, we have TIR and Endplay tolerances on total encoder movement after it is mounted.

The reason for this is because unlike modular incremental encoders, which rely on the mounting shaft to hold the disk and sensor air gap, the QD145 series of incremental encoders has an internal bearing set that maintains the air gap. This takes the need for an exacting precision shaft to shaft fit out of the list of problems motor manufacturers face when designing a new motor.

I called the customer back and let him know the good news; that instead of some tight machining numbers, he only needed to keep his QD145 Incremental encoder within .007” of radial shaft runout and within +/- .030” for axial shaft runout.

Quantum Devices is now offering an interface cable to our QDH20 series of Industrial Incremental Encoders.

While we like to focus more on building encoders than “add on” products, we did find this to be something that made sense to offer with our QDH20 encoders.

At this time the ten pin Female MS connector to flying leads is the only interface cable for the QDH20 encoder that is option being offered. Part numbers can be constructed by using the prefeix 2100A and adding the length of the desired cable in inches as a part suffix.

Example:

If an eight foot cable is desired, 8 x12 = 96 so the part number would be 2100A096

I recently received an e-mail asking for some help picking the right resolution for an Optical Incremental Encoder. I have removed the personal information and the drive/controller information, otherwise the e-mail is verbatim:

I think I see what you are after, but you are missing some information. The Encoder calculator on the web site is a bit too simple for what you are trying to do. It is really meant to be a quick conversion tool to find out of your Incremental Encoder is going to violate controller input frequency limitations, and the like.

The ability of the system to regulate to a given speed will depend on more than just the line count of the encoder. The real question that needs to be answered before we can determine Incremental Encoder resolution is “How many pulses are needed per update?” this is a question that will need to be asked of the Drive manufacturer/supplier, but I am sure they will have further questions about the motor size and loading as well.

It is easy to see that a motor without a load is easier to regulate than one with a dynamic (changing) load. Therefore the size of the motor (and it’s inertia) and the size of the load (and it’s inertia) will need to be taken into account.

With all that being said, we can do some quick math to get a feel for what we do know about the application.

Before I get started, one major thing to note here is that it appears that you are asking for regulation to occur on the output of the gearbox. I am guessing that the Incremental Encoder is on the motor side (input) of the gear box, so there will be a scaling factor of the input to output ratio that will need to be taken into account along with the following math:

We know that you want to regulate speed within a range of 0.1 RPM to 230 RPM.

After this we really need to ask is “What can the drive do?” At the top speed we will travel over a degree before the drive can update the loop for any error component. In the minimum speed example, the drive will not see a change in count over 1667 loop updates. How does the drive handle this?

To see what this means in Incremental Encoder pulses vs Drive Loop Updates we can take the highest line count encoder we currently provide, a 20,000 LC QR12:

Just taking a very rough look at it, a 20,000 line count incremental Encoder appears to have more than enough data per update to give the drive a good idea of how fast the motor is turning at the high speed, while at the slow speed the drive has to wait for several loop updates to pass until it sees even one edge of the Incremental Encoder Signals.

Will the drive be able to regulate at this lower speed without falling out of the 1% tolerance? My guess is probably not, but only the drive manufacturer can answer that for sure. I am also wondering if 1% regulation is really needed at these slow speeds.

Keep in mind that these numbers would also need to be scaled by the input to output shaft ratio if the encoder is on the input side to the gearbox and the RPM’s above are actually referring to the output shaft speed.

Can the application tolerate being out of the 1% specification momentarily while the drive recovers? If so, your focus should likely be on loop response time.

At the very least I am guessing tuning the loop to accommodate for both full speed and slow speed may be difficult.

Why do my incremental encoder signals disappear?

Today I was called to the production floor after one of the test Technicians noticed that the encoder signals disappeared from the oscilloscope when they ran the encoders “unloaded”.

I asked one of the techs if the encoders being tested were of the open collector variety. The answer was “No, they’re line drivers”

I immediately pictured our standard 26C31 line drivers, which are push-pull devices.

When I checked the label on the line driver IC on the encoder. I saw that it was an ET7273. This meant that the right answer was that they were both a line driver and an open collector device. The ET7273 is used in our optical encoder when a customer wants complementary open collector outputs, or incremental side open collector outputs

The problem was that I, as well as the Tech I talked to, am guilty of using the term “line driver” to synonymously mean “push pull” outputs.

Open collector style outputs are used when you want to be able to pull up to a different voltage level from the rest of the system. For example most of the optical encoders we sell are 5V devices, but the open collector output can be pulled up to many times that level. The ET7273 has a max spec of 30Vdc.

Lets say you needed to identify where the index pulse was firing on your incremental encoder, but you left your oscilloscope in your other jacket pocket, and now all you have on hand is a DMM.

Well fear not, finding the index with a multimeter is possible although a bit tedious.

The index fires once per revolution and at higher line counts this makes it VERY easy to miss. Since there is some delay in a multimeter’s display time, you will need to rotate the encoder very slowly to catch a change in voltage level.

The Blue box has a nine-volt battery inside that I regulated down to 5Vdc for the encoder power. I have pulled out connections to ground (Black wire) and the index channel (Orange wire). When the index fires, the voltage will go from zero to five volts.

This wire is intended to be used in situations where the encoder flex mount is not case grounded. The Black/white wire is at the same potential as the Optical Encoder’s conductive polymer housing and flex mount.

Internally the electrical path of the black-white wire is tied to the optical encoder housing and flex mount through the circuit board. Our optical encoders tend to live on the back of hardware like Brushless DC motors, where the motor housing is at ground potential. In cases like this it is usually best to leave the black-white wire tied off and floating.

Arrow showing flex mount grounding Optical Encoder to Motor.

Jim is an application engineer for Quantum Devices INC, a leading manufacturer of optical encoders.